That would be a great answer if he didn't say his RIGHT brake light didn't work. Since he specified only ONE bulb, the answer is more likely than not that he has a bad bulb. Since they're dual filament style he's probably lost the filament that takes more energy and turns on and off more often, which would be the BRAKE filament. Fuses aren't that likely because again it's only ONE side. It could be wiring but it's not likely because his tail and running lights work. He needs to go buy a new $2 light bulb at Autozone and replace it to see if he has the same problem. If he does he should work back from there. If he can't figure out how to do that, he should probably go see a Mechanic and just give him $90 for an hour of time to fix it, because he'll probably just mess it all up worse.